The present invention relates to towed arrays used for underwater sound detection, and more particularly to an array which is characterized by reduced turbulence and acceleration noise.
The usefulness of towed arrays for underwater sound detection is limited by the noise in the system. The instrument noise background (which determines the ultimate sensitivity of the array) is caused to a large extent by 1) cross talk between individual hydrophones, 2) turbulence caused by the towing of the array cable through water, and 3) acceleration also caused by the towing.
Conventional towed arrays use piezoelectric sensors for detecting pressure variations due to acoustic waves. Such sensors are electrical and, therefore, sensitive to electromagnetic interference (EMI). It is also difficult to isolate the output of individual hydrophones; therefore, there is considerable cross talk between the sensors.
Fiber optic towed arrays are known, wherein a number of acoustic sensor elements such as fiber optic hydrophones are series connected as shown in FIG. 1. A laser 10 directs an input light pulse of length T into the optical fiber 12. The array hydrophones are Mach-Zehnder interferometric sensors formed by the sensing fiber optic coils S.sub.1, S.sub.2 . . . S.sub.N and the reference coil 16, where .tau. is the delay time through each coil. The optical pulse train from the output fiber 14 carries pressure information sensed by the sensing coils. Such a system is described in further detail in "Characterization of an Eight-Element Fiber Sensor Array," A. D. Kersey et al., SPIE Vol. 985, Fiber Optic and Laser Sensors VI (1988). While such a system avoids the EMI problem, it is still difficult to isolate the output of different sensors, and the array still suffers from the background noise caused by towing turbulence and acceleration. Moreover, such a system is subject to intrinsic optical crosstalk which arises due to multiple couplings of optical pulses between the serial sensor chain and the output collection bus fiber.
To detect weaker sound sources, the self noise of the array has to be reduced. The self noise is caused in conventional towed arrays primarily by the speed of the array through the water. The speed causes turbulence and acceleration which appear as sound in the hydrophones that constitute the array, i.e., the apparent high background noise that has nothing to do with the target's sound present in the water (sea).
It is therefore an object of the present invention to provide a fiber optic towed array having high sensitivity and characterized by substantial reduction in self noise and by elimination of crosstalk among the sensors.
A further object is to provide a method for paying out a towed array from the towing vessel or vehicle in a manner so as to minimize the turbulence caused by the array moving through the water.